There’s a powerful device out there collecting huge amounts of detailed information about your lifestyle, location, and habits—and it’s tucked away right in your pocket. Each smartphone is uniquely differentiated from hundreds of millions of other smartphones by something called an ad ID. Google’s version is known as GAID (Google Advertiser Identification) and Apple’s is called IDFA (Identifier For Advertisers).

This technology, in its various forms, allows brands, carriers, ad tech companies, and others to collect valuable insights about you—from your spending habits at Starbucks, to your Tinder matchups, or even last night’s bar tab. Data gathered from advertiser IDs are also bought and sold many times over and may include some personal details.

This exchange of personal information is increasingly becoming the standard, and yet many of us are unconcerned. A Pew Research study found that many people are willing to give out private information as long as they receive something of value in return. Millennials are especially receptive. A recent USC study found that one in four Millennials are willing to disclose personal information in exchange for more relevant advertising. The popularity of store membership cards, like My Starbucks Rewards, has proven that consumers are willing to sacrifice some privacy in exchange for convenience, rewards, or promotions.